The present invention relates in general to communication systems, and is particularly directed to a circuit for protecting the electronic circuitry of telephone equipment, such as but not limited to a telephone test set, from an excessive current condition such as may occur, if the telephone ring and tip leads become directly coupled to a low impedance path to a high voltage source, such as battery or loop feed circuit.
The fact that manufacturers of telephone equipment provide instruction manuals and guidelines for use and maintenance of their equipment is no guarantee that users will not subject such equipment to potentially damaging operational and/or environmental conditions. Indeed, it is often the case that telephone repair technicians connect their portable test set equipment to very high voltage and current sources, such as power supplies or telephone loop feed circuits having very low series resistances and/or high voltages. When this happens, loop currents considerably in excess of the rated value (e.g., on the order of 100-150 ma) may result. Prolonged operation of the equipment in such a high current condition will eventually lead to failure of the equipment""s electronic circuitry.
One proposal to deal with the high current condition has been to switchably interrupt or insert a high impedance device in the loop current path to the protected circuitry. While this overcurrent intervention approach prevents damage to the circuit to be protected, it also effectively renders the telephone equipment inoperative until the high current condition has terminated.
Rather than effectively shutting down the telephone equipment until the overcurrent condition subsides, the invention employs selectively controlled high current protection circuit that is effective to maintain sufficient current flow for continuous operation of the telephone equipment""s functional circuitry, while at the same time controllably by-passing excess and/or reducing the amount of loop current being drawn into the equipment, that could potentially damage the protected circuitry.
For this purpose, the over current protection circuit of the invention comprises a series current-limiting element installed in the loop current flow path between a tip and ring interface (bridge rectifier) and the circuitry of the telephone equipment to be protected. An overcurrent current shunting or by-pass circuit is installed across the tip-ring ports of the protected circuit downstream of the current-limiting element. This overcurrent by-pass circuit may include the collector-emitter path of a bipolar transistor, the base of which is coupled through a level-shifting and amplifier transistor to a current sense circuit.
The current sense circuit may include a current sensing bipolar transistor, which is controllably turned on by the voltage across a relatively low value current sense resistor installed in the loop current path through the telephone""s electronic circuitry to be protected. By relatively low value resistance is meant one that does not affect the operational performance of the circuitry to be protected.
During normal operation, the current flow through the current sense resistor will be less than that sufficient to forward bias the current sense transistor, so that the shunting circuitry is turned off, and all of the loop current flows through the tip-ring path, including the series current-limiting element, the protected circuitry and the current sense resistor. If the tip-ring terminals of the telephone (test) set are coupled to a high current source, such as being placed directly across the terminals of battery, then the current flow through the current sense resistor will increase to a value sufficient to turn on the current sense transistor. This, in turn will cause the overcurrent by-pass transistor to divert or by-pass a portion of the current that would otherwise flow through the electronic circuitry by way of the tip-ring loop current path.
For a medium magnitude overcurrent condition (e.g., on the order of 100-150 ma), this by-pass operation is sufficient to allow useful loop current (e.g., on the order of 20-100 ma) to flow through the protected circuitry, while allowing the excess current to be diverted without damaging the current-shunting components. However, if a substantial overcurrent condition persists for an extended period of time, the resulting power dissipation associated with this high current flow will cause the series high impedance element to change from its low impedance state to its high impedance state. In this high impedance state, sufficient operational loop current (e.g., 20-30 ma) will continue to flow through the protected circuitry, yet the components of the shunting circuitry will be protected. Once the overcurrent condition terminates, the series element will revert to its low impedance state.